Experiment was conducted with several breeds and their crossbreds to analyze the differences of their normal growths and those under restricted diet by which their increases of body weights were interrupted for a week. And the patterns of recovery of growth under full diet following the restrictions were also studied.
Experiment was carried out with 185 cockerels hatched on July, 1963. Chickens used came from cockerels of four pure breeds; White Leghorn (WL), Barred Plymouth Rock (BPR), White Cornish (WC) and White Rock (WPR), and their crossbreds; WC _??_ _??_ × WL _??_ _??_ , WC _??_ _??_ ×BPR _??_ _??_, and WC _??_ _??_ ×WPR _??_ _??_, The experimental groups were fed with the restricted diet for a week starting at a week of age, and thereafter with full diet. The control groups were fed
ad lib. consistently. Six cockerels from the control groups were chosen at random and dissected at ten weeks of age. Body weight, food consumption, feathering, keel length and length of metatarsal bone were measured.
1) The average body weights of WC and WPR were 1,586gms. and 1,528gms., respectively, at 10 weeks of age, and were larger than those of the other breeds. The body weight of F
1 showed intermediate between their parents. Heterosis over midparent value was observed in WC×WL. More efficient feed conversion at 10 weeks of age was obtained in cockerels of the heavier breeds. The ratios were 3.48 in WL, 2.88 in BPR, 2.85 in WPR, 2.84 in WC×WPR, 2.83 in WC×WL, 2.75 in WC×BPR and 2.73 in WC, respectively. Two different types of the ratios were observed between WC×WL and WC× BPR after the period of the restriction of diet. These differences might be resulted from genetical differences of their breeds.
2) Length of keel at ten weeks of age was longer in the heavy breeds than in the lighter. In general, the longer the keel length, the more meat production. Consequently, in this experiment, dressing percentage was higher in the heavy and their F
1 breeds than in the other breeds. BPR and their F
1 had a defect of late feathering though they had the highest percentage of dressing.
3) No difference ameng light (WL), intermediate (BPR), Heavy breed (WC, WPR) and their F
1 cockerels was observed with the effect of the restriction and of recovery from the restriction upon growth rate despite of the difference in their growth patterns.
4) Since the length of metatarsal bone and second primary flight develop at earlier age, the effect of the restricted diet on both characters was not so much as those on live weight. Returning from the restriction to full diet, the recovering rate of live weight immediately increased, while those of metatarsal bone and second primary flight increased a week after from full feeding. This fact may be regarded to indicate that viscera and muscle which are major conformation of live weight recover from the influence of the restricted diet prior to the other characters.
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